1. Field of the Invention
The present invention relates to an epoxy resin composition for sealing a semiconductor, and more particularly, it relates to an epoxy resin composition for sealing a semiconductor, in which a novel copolymer is employed as a flexibilizer.
2. Description of the Background Art
In recent years, a semiconductor element has been increased in chip area and reduced in resin layer thickness, following high density integration. When a semiconductor part is sealed with a conventional epoxy resin composition in a fabrication step, it easily causes problems such as cracking of the chip, disconnection of a bonding wire, sliding of an aluminum wire, cracking of sealing resin, and the like, which are critical defects for the semiconductor part. This is because the conventional epoxy resin composition for sealing a semiconductor has been mainly developed in consideration of improvement in heat resistance and moisture resistance, and a hardened body thereof is so inferior in flexibility that significant stress is applied to the element.
As to reduction of stress in an epoxy resin composition for sealing a semiconductor, generally known are a method of decreasing a thermal expansion coefficient of resin for reducing thermal distortion, and a method of decreasing an elastic modulus for reducing stress which is caused by thermal distortion. In order to increase a temperature region having small thermal distortion while maintaining heat resistance and moisture resistance, it is necessary to increase a glass-transition temperature.
The aforementioned stress, which is caused by thermal distortion, can be reduced by addition of a flexibilizer. However, in a method of reducing the elastic modulus by blending a generally used flexibilizer such as long-chain alkylene polyamine, polyoxyalkylene glycol or bisphenol A diglycidyl ether having long-chain alkylene oxide, for example, the glass-transition temperature of a hardened body is significantly reduced, to cause reduction in heat resistance and moisture resistance (refer to Japanese Patent Publication No. 59-8718, Japanese Patent Laying-Open Nos. 59-30820 and 59-226066, etc.).
On the other hand, an elastomer denatured flexibilizer obtained from polybutadiene having functional groups which are reactive with epoxy resin on both ends, a copolymer of butadiene and acrylonitrile or the like has been also considered as a flexibilizer which causes merely small reduction in moisture resistance and glass-transition temperature (refer to Japanese Patent Laying-Open No. 58-174416, Japanese Patent Publication No. 58-108220, Japanese Patent Laying-Open No. 58-184204, Japanese Patent Publication No. 62-9248, Japanese Patent laying-Open Nos. 59-113021 and 59-58024, etc.). In such an elastomer denatured flexibilizer, however, unsaturated bonds contained in the elastomer are oxidized and deteriorated under a high temperature, leading to a problem of disappearance of the flexibilizing effect.
Also known is a method of dispersing silicone resin or silicone rubber having a low elastic modulus, which is an excellent flexibilizer in view of electric characteristics and thermal stability under a high temperature (refer to Japanese Patent Laying-Open Nos. 62-84147, 56-4647 and 64-29450 etc.). However, moisture permeability of a hardened body obtained by this method is increased since silicone resin is inferior in adhesion with a metal while silicone rubber is inferior in interfacial strength with respect to an epoxy matrix. Thus, this method is inferior in reliability due to poor moisture resistance and small mechanical strength.
The inventors have already found that an epoxy resin composition employing a pre-reactant of denatured silicone oil having epoxy groups and phenol novolak resin can provide a hardened body which has heat resistance and moisture resistance, as well as a low elastic modulus (refer to Japanese Patent Publication No. 62-83158). They also have found that a pre-reactant of denatured silicone oil having hydroxyphenyl groups and epoxy resin provides a hardened body which has heat resistance and moisture resistance, as well as a low elastic modulus, a low thermal expansion coefficient and a high glass-transition temperature (refer to Japanese Patent Application Nos. 63-115269 and 63-161849).